Trellis-coded modulation (TCM) combines channel coding and modulation at a transmitter. Information bits are sent into a TCM encoder in parallel. The least-significant bits (LSBs) are convolutionally encoded. The most-significant bits (MSBs) are usually left uncoded and combined with the output of the convolutional coder to form a symbol on a signal constellation through a signal mapper. One advantage of TCM is that although no error control coding is performed on any bit other than the LSB of the input information data, the decoder is able to provide error correction on all bits. Therefore, significant coding gain over uncoded modulation can be achieved.
In eight-state phase shift keying (8PSK) modulation, a modulated carrier signal is transmitted with symbols that each represent three data bits. A difficulty with PSK transmission is that the modulated carrier as transmitted can be affected by undesirable phase rotation. At the receiver, such phase rotation corresponds to a rotation of the received signal constellation such that decoded symbols do not match those which were transmitted.
It is known to provide resolution of the phase ambiguities for 2/3, 5/6 and 8/9 trellis coded 8PSK modulation schemes in which the modulation is based on two coded bits per symbol. Such known approaches enable 90.degree., 180.degree. and 270.degree. phase ambiguities to be resolved.